Field of Invention
This invention relates generally to magnetic recording media, and more particularly to thermally stable high density media.
Description of Related Art
Modern magnetic recording media reaches ever higher recording densities. Further increase of the areal density is believed to be limited by the superparamagnetic limit. This limit represents that as the size of the magnetic grains in the media decrease, at some grain size the thermal fluctuations at room temperature kBT300 become capable of overcoming the energy barrier ΔE=KV, which separates the two magnetization directions of an isolated grain with a volume V and an uniaxial anisotropy constant K. This superparamagnetic limit, or thermal instability, can be overcome by increasing the anisotropy K, according to the Stoner-Wohlfarth theory. However, such an increase also results in an unfavourable increase of the coercivity Hc. As a consequence, these grains are thermally stable but can not be written with existing recording heads.
Various improvements have been proposed to counter this thermal instability recently, also known as the writeability problem. In U.S. Pat. No. 6,468,670 a continuous ferromagnetic overlayer was introduced to increase the Signal to Noise Ratio (SNR). U.S. Pat. No. 6,280,813 and U.S. Pat. No. 6,383,668 addressed the thermal instability problem by replacing the conventional single magnetic recording layer with two ferromagnetic films that are antiferromagnetically coupled together across a nonferromagnetic spacer film, and a ferromagnetic layer that is coupled to a synthetic antiferromagnet, respectively. This idea reduces the demagnetizing field of the bits in the case of longitudinal magnetic recording. U.S. Pat. No. 5,583,727 proposed to overcome the problem by employing thermally assisted recording. In the paper “FeRh/FePt exchange spring films for thermally assisted magnetic recording media” Applied Physics Letters, Vol. 82, Issue 17, April 2003, pp. 2859-2861, Thiele et al. suggested to lower the coercive field by the use of FePt/FeRh bilayer system. The proposed architecture included a hard layer, exchange coupled to an antiferromagnetic layer. After heating the antiferromagnetic layer across a transition temperature, it became ferromagnetic with a large magnetic moment and low magnetocrystalline anisotropy. Thus, upon crossing the transition temperature the antiferromagnetic layer acted as a magnetic soft layer that helped to reverse the hard layer.
In the paper “Composite Media for Perpendicular Magnetic Recording”, IEEE Transcations on Magnetics, Vol. 41, No. 2, February 2005, pp. 537-542, R. H. Victora and X. Shen proposed magnetic multilayer structures composed of magnetically hard and magnetically soft layers. In the model of Victora and Shen, the magnetization of the soft and the hard part of each grain remained uniform. In order to decrease the coercive field, the exchange coupling between these layers had to be reduced with a decoupling layer. Motivated by the theoretical work, Wang et al. performed an experimental work on two layer composite media. The results were reported in “Composite media (dynamic tilted) media for magnetic recording”, Applied Physics Letters, Vol. 86, April 2005, pp. 142504. Wang et al. concluded that a coupling layer was required in composite media to decrease the exchange coupling between the soft and hard layer, in accordance with the theory. This was in contrast to the paper “Exchange spring media for perpendicular recording,” Applied Physics Letters, Vol. 87, July 2005, pp. 12504-12507 by Suess et al., incorporated herein by reference in its entirety, where states with inhomogeneous magnetization were formed.
The paper “Preliminary Study on (CoPtCr/NiFe)—SiO2 Hard/Soft-Stacked Perpendicular Recording Media”, IEEE Transactions on Magnetics, Vol. 41, No. 10, October 2005, pp. 3136, Y. Inaba et al. considered a sufficiently thin soft magnet coupled to a sufficiently thin hard magnet in order to keep the magnetization uniform and parallel in both layers during reversal. The paper “Exchange spring recording media for areal densities up to 10 Tbit/in2”, Journal of Magnetism and Magnetic Materials, Vol. 290-291, 2005, pp. 551-554 (available online 18 Dec. 2004) by Suess et al., incorporated herein by reference in its entirety, proposed a tri-layer structure which was composed of a hard layer at bottom, a soft layer in the middle and a hard layer on top.
The paper “Exchange spring media for perpendicular recording,” Applied Physics Letters, vol. 87, 30. June 2005, pp. 012504, by Suess et al., incorporated herein by reference in its entirety, domain wall assisted recording on bilayers was presented. Subsequent work by A. Dobin and H. J. Richter (presented at the Intermag conference 2006, talk DB-10, San Diego, Calif., May 2006; preprint available at http://arxiv.org “Domain Wall Assisted Magnetic Recording” by Dobin and Richter) followed the same approach.
In previous works it was not shown that a finite value of the anisotropy in the soft magnetic layer does not reduce the thermal stability of the structure. This question needs to be investigated, as larger anisotropies in the softer layer reduce the energy that is required to push a domain wall from the soft layer to the hard layer. Further, typical multilayer exchange spring media do not even contain a soft magnetic layer. Instead, it contains a nucleation layer which can be magnetically hard. The softest layer in the nucleation layer can have a coercive field similar to typical fields of recording heads.
For all these reasons, the choice of layer architectures and their anisotropies to overcome the superparamagnetic limit in optimal fashion remain a topic of intense investigations.